Austin City Limits wrapped up its first of two weekends last Sunday, and the night before The National took the stage at Zilker Park they treated their fans not only to an intimate performance at local BBQ outpost Stubb’s, but to a special guest as well: Justin Vernon. The Bon Iver frontman had finished up a set with his blues-rock side project, the Shouting Matches, earlier in the evening and joined the band on guitar and vocals for “Terrible Love,” and a passionate singalong of “Vanderlyle Crybaby Geeks,” both off 2010’s High Violet (to which Vernon actually contributed backing vocals).